Method for nitriding suspension components

ABSTRACT

A controlled nitrogen diffusion process is employed on a steel coil spring of a suspension system. One known process is that known as the Nitreg® process. Ammonia is introduced into a furnace atmosphere and nitrogen diffuses into the coil spring, creating a hardened diffusion zone which resists fatigue and fractures. The Nitreg® nitrogen diffusion process is a computer controlled menu driven process which continually monitors the process parameters. By regulating the nitriding potential, the monitored process parameters are automatically adjusted to achieve optimal results. The nitriding potential is the tendency of nitrogen to be absorbed by steel and is expressed as the ratio of the partial pressure of ammonia to the partial pressure of hydrogen. Additionally, the depth of the white compound layer can be regulated or eliminated, reducing distortions.

[0001] This application claims priority from provisional applicationserial No. 60/266,350 filed Feb. 2, 2001.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to a method for nitridingsuspension components.

[0003] Vehicles are commonly equipped with suspension systems forabsorbing road shock and other vibrations, while providing for a smoothand comfortable ride. Steel coil springs are utilized as a part of thevehicle suspension system. The coil springs must be able to resiststresses which cause fractures and decrease the fatigue life of the coilspring.

[0004] A nitriding process has been utilized to improve the fatigue lifeof a coil spring utilized in a vehicle suspension system. Nitridingproduces compressive residual stresses on the surface of the coil springwhich counteract the tensile stresses produced by everyday use whichcause fractures.

[0005] Additionally, the nitriding process forms a white layer on theexterior surface of the steel. For most applications, this white layerhas no useable properties. The layer is very hard, but brittle, and mayspall during use. As it has no useable properties, it is often removedby treatment grinding or finishing.

SUMMARY OF THE INVENTION

[0006] A controlled nitrogen diffusion process is employed on theexterior surface of a steel coil spring of a suspension system to createa hardened layer which reduces fractures and improves fatigueproperties. One known process is the Nitreg® process. The Nitreg®process is a computer controlled menu driven process which regulates thenitriding potential of the furnace atmosphere. The nitriding potentialis the tendency of nitrogen to be absorbed by steel and is expressed asthe ratio of the partial pressure of ammonia to the partial pressure ofhydrogen. Electric sensors and furnace components balance the nitridingatmosphere required to maintain the desired nitriding potential.

[0007] The steel coil spring is first cleaned to remove scale from theexterior surface. In an atmospheric furnace, the coil spring is heatedand ammonia is released. Nitrogen from the ammonia diffuses into theexterior surface of the steel coil spring, creating a hardened diffusionzone on the coil spring. After cooling the coil spring, the coil springis subject to shot peening to instill high compressive residual stresseson the surface of the coil.

[0008] The nitriding potential of the furnace atmosphere is regulateddepending on the type of steel utilized and the applicationrequirements. By regulating the nitriding potential, the depth of thediffusion zone can be controlled. Additionally, the depth of the whitecompound layer can be regulated or eliminated to reduce distortions.

[0009] These and other features of the present invention will be bestunderstood from the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The various features and advantages of the invention will becomeapparent to those skilled in the art from the following detaileddescription of the currently preferred embodiment. The drawings thataccompany the detailed description can be briefly described as follows:

[0011]FIG. 1 illustrates a schematic side cross sectional view of anitrided steel coil spring;

[0012]FIG. 2 illustrates a graph relating the hardness of various steelsto the depth below the surface for a prior art nitriding process; and

[0013]FIG. 3 illustrates a flowchart of the controlled nitrogendiffusion process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] This invention relates to a method for nitriding a coil spring ofa suspension system utilizing a controlled nitrogen diffusion process.One known process is the Nitreg® process. The Nitreg® nitrogen diffusionprocess is a surface hardening heat treatment wherein the nitridingpotential of the furnace atmosphere is regulated depending on the typeof steel utilized and the application requirements.

[0015]FIG. 1 illustrates a side cross sectional view of a nitrided steelcoil 8. Nitrogen is introduced into the surface of the steel at aspecific temperature range while in the ferritic condition to harden thesurface of the steel. Nitrogen is partially soluble in iron. At nitrogencontents up to approximately 6%, nitrogen forms a solid solution withferrite. When the nitrogen content reaches about 6%, a white layerhaving a composition of Fe₄N is formed. As shown in FIG. 1, this layeris the compound layer A. The compound layer A is very hard, but isbrittle and may spall in use.

[0016] When the nitrogen content reaches 8%, the equilibrium reactionproduct Fe₃N is formed, illustrated as diffusion zone B in FIG. 1. Thediffusion zone B is hardened by the formation of the Fe₃N compound. Thediffusion zone B is the layer which provides surface hardening. Belowthe diffusion zone B is steel zone C. Steel zone C is the inner steelportion of the coil spring in which there is no nitrogen diffusion. Thehardness of various steels at the different zones are illustrated inFIG. 2.

[0017] Nitrided steels generally contain strong nitride-forming elementssuch as aluminum, vanadium, molybdenum, titanium and chromium. Whenthese steels are nitrated, the nitride-forming elements form particleswith the nitrogen which create strengthening dislocations by strainingthe ferrite lattice.

[0018] The Nitreg® nitrogen diffusion process of the present inventionis employed on a steel coil spring 8 to create a hardened layer andimprove fatigue properties. The Nitreg® nitrogen diffusion process is acomputer controlled menu driven process which hardens the surface of thecoil spring 8 by regulation of the nitriding potential of the furnaceatmosphere. The nitriding potential is the tendency of nitrogen to beabsorbed by steel and is expressed by the ratio of the partial pressureof ammonia to the partial pressure of hydrogen. Electronic sensors andfurnace components help balance the nitriding atmosphere to maintain thespecific nitriding potential. The sensors continually monitor and adjustthe process parameters to regulate the nitriding potential. Thenitriding potential is programmed depending on the type of steelutilized and the application requirements.

[0019] The nitriding process 10 is illustrated schematically in FIG. 3.The coil spring 8 is first cleaned 12 to remove scale from the exteriorsurface. The scale is removed either by shot peening or shot blasting.The exterior scale can also be removed by utilizing hydrochloric acid onthe surface of the coil spring 8. The coil spring 8 is positioned in anatmospheric furnace. In the furnace, the coil spring 8 is heated 14 to atemperature between 380° C. and 480° C. After heating 14, ammonia isreleased 16 into the furnace for approximately 3 to 8 hours.

[0020] The amount of ammonia released 16 depends on the desirednitriding potential.

[0021] Nitrogen from the ammonia diffuses 20 into the exterior surfaceof the steel coil spring 8, creating a hardened diffusion zone B. Byregulating the nitriding potential 18 by continually monitoring theprocess parameters with sensors, the depth of the diffusion zone B canbe controlled. The diffusion zone B is preferably between 30 μm and 100μm deep. Once the nitrogen from the ammonia has diffused into the steelcoil spring 8, the coil spring 8 is then cooled 22. The entire cyclelasts approximately 12 to 20 hours.

[0022] By regulating the nitriding potential 18 depending on the type ofsteel utilized and the application requirements, the white compoundlayer A can be controlled or eliminated. In traditional nitriding, theprocess produces uncontrollable white layer growth which is removed bypost treatment grinding or finishing. The Nitreg® process produces ahard and non-spalling white compound layer A on the exterior surface ofthe coil spring 8. The growth of the compound layer A can be controlledby regulating the nitriding potential. In the preferred embodiment, thecompound zone A is 0 to 2 μm thick.

[0023] After the nitriding process, the coil spring 8 is subject to ashot peening process 24 to instill high compressive residual stresses onthe surface of the coil spring 8.

[0024] Preferably, a two-step process is employed. In the first step,the first peening is done with a 0.8μmm diameter shot, and in the secondstep, the second peening is done with a 0.3 mm diameter shot. Byemploying the additional shot peening step, there is at least a six foldincrease in the fatigue life of the coil spring 8 having a 0.5 inchdiameter wire subjected to a stress of 564 +/−476 Mpa.

[0025] The Nitreg® nitrogen diffusion process provides surfacehardening. The fatigue properties of the coil spring 8 can be improved,allowing for higher stress design and/or lighter weight springs. Theprocess strengthens the exterior surface of the coil spring 8,decreasing fractures and increasing the fatigue life. Additionally, asthe process is computerized, the results are repeatable.

[0026] The foregoing description is only exemplary of the principles ofthe invention. Many modifications and variations of the presentinvention are possible in light of the above teachings. The preferredembodiments of this invention have been disclosed, however, so that oneof ordinary skill in the art would recognize that certain modificationswould come within the scope of this invention. It is, therefore, to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specially described. For that reasonthe following claims should be studied to determine the true scope andcontent of this invention.

What is claimed is:
 1. A method for surface hardening a steel coilspring of a suspension system comprising the steps of: nitriding asurface of said coil spring; and regulating a nitriding potential in anitriding atmosphere to control the step of nitriding said coil spring,said nitriding potential being the tendency of nitrogen to be absorbedby said steel coil spring.
 2. The method as recited in claim 1 whereinthe step of regulating said nitriding potential further includesmonitoring at least one process parameter.
 3. The method as recited inclaim 1 wherein the step of nitriding said coil spring further includesintroducing ammonia into said nitriding atmosphere.
 4. The method asrecited in claim 1 further comprising the steps of: cleaning saidsurface of said coil spring; heating said coil spring; and cooling saidcoil spring.
 5. The method as recited in claim 4 wherein the step ofheating said coil spring includes heating said nitriding atmosphere to atemperature between 380° C. and 480° C.
 6. The method as recited inclaim 1 wherein the step of nitriding said coil spring produces adiffusion zone having a depth between 30μm and 100μm.
 7. The method asrecited in claim 1 wherein the step of nitriding said coil springfurther includes forming a compound layer on said surface of said coilspring having a depth between 0 and 2μm.
 8. The method as recited inclaim 1 further comprising the step of shot peening said surface of saidcoil spring.
 9. The method as recited in claim 9 wherein the surface ofsaid coil spring is shot peened with a 0.8 mm diameter shot and a 0.3 mmdiameter shot.
 10. A method for surface hardening a steel coil spring ofa suspension system comprising the steps of: cleaning said surface ofsaid coil spring; heating said coil spring; nitriding a surface of saidcoil spring; regulating a nitriding potential in a nitriding atmosphereto control the step of nitriding said coil spring, said nitridingpotential being the tendency of nitrogen to be absorbed by said steelcoil spring; cooling said coil spring; and shot peening said surface ofsaid coil spring.
 11. A steel coil spring of a suspension systemcomprising: a steel body portion having a surface; and a diffusion zoneproduced by nitriding said surface of said coil spring by regulation ofa nitriding potential.
 12. The coil spring as recited in claim 11wherein said surface of said coil spring is nitrided by introducingammonia into a nitriding atmosphere.
 13. The coil spring as recited inclaim 11 wherein a nitriding atmosphere is heated to a temperaturebetween 380° C. and 480° C.
 14. The coil spring as recited in claim 11wherein said diffusion zone has a depth between 30μm and 100μm.
 15. Thecoil spring as recited in claim 11 wherein said coil spring furtherincludes a compound layer having a depth between 0 and 2μm.